This invention relates to gas fired radiant burners. More particularly it relates to gas fired radiant burners made of ceramic materials.
Heat energy is normally transmitted by conduction, convection, or radiation. In many applications it is desirable to utilize radiation as the primary means for transmitting heat. Radiant energy is not affected by the movement of air, may be directionally controlled and focused, and the intensity may be readily controlled, thereby enabling higher efficiencies than convection or conduction transmissions.
It is also often desirable to utilize natural gas as an energy source for producing heat. Natural gas is abundant and is one of the most environmentally clean sources of energy. Natural gas fired infrared heat generators are often referred to as radiant burners. These radiant burners generally include radiant burner plates or radiant burner tubes which are porous so as to permit the gas to pass therethrough. Natural gas and air are mixed in a plenum which is connected to the radiant burner plate or tube. In some cases the combustion mixture of air and gas is conveyed through holes in the burner plate and the gas burns above the surface of the plate. In that case the surface is heated by conduction from the close proximity of the flame. In other cases, the flame occurs below the surface of the plate which is heated directly by the gas flame. In other cases the heating of the plate occurs both at the surface and within the porous structure so that there is a combination of conductive heating and direct flame heating of the plate. Often the plate is made of a ceramic material.
A typical commercially available porous ceramic type radiant burner assembly 10 is shown in FIG. 1. A plenum 12 receives an air and gas mixture through orifice 14. A solid ceramic plate 16 forms the top of the burner assembly 10. Burner plate 16 includes a plurality of holes 18 which communicate with the inside of the plenum 12. The gas passes through the holes 18 and is ignited at the surface 20 of burner plate 16. The surface 20 of burner plate 16 is somewhat of a wavy construction so that there are alternate peaks and valleys. This type of burner plate is referred to as a ported tile.
There are also gas fired radiant burner plates made of reticulated ceramic foam such as that described in U.S. Pat. Nos. 3,912,443. 4,643,667 also teaches the use of a ceramic porous plate, which appears: to be foam, as a gas fired radiant burner. It has been found that the use of reticulated ceramic foam has many advantages over the ported tiles shown in FIG. 1. The primary advantage is that the foam is a more efficient radiating surface so that more heat is absorbed from the flame and converted into radiant energy as evidenced by the higher surface temperature of the foam. It is believed that this occurs primarily because the reticulated materials have substantially more surface area than the ported tile.
One of the primary problems with the use of ceramic foams is that the temperature of the surface tends to be uneven. The flame should burn just under the top (radiating) surface of the foam so that the heat is transferred both to the surface of the foam by conduction as well as by direct contact with the flame. If some of the pores are blocked, the air/gas mixture does not reach that particular portion of the surface and cold spots are the result. If the pores in the foam structure are too open or if the pores are too small, the flame will burn off the surface again resulting in a cold spot because of insufficient conduction at that position on the surface of the plate.
There have been attempts to solve this problem by laminating foams of two different pore sizes together. This technique is taught in both U.S. Pat. Nos. 3,912,443 and 4,643,667 which are referred to above. Generally, the bottom layer is made of a fine pore foam, for example 30 to 100 pores per inch, and the top layer is made of a coarser foam, for example from 5 to 20 pores per inch. This causes the flame to burn at the surface of the fine foam but within the layer of the coarse foam so that the coarse foam, which is the radiant burner plate, is heated directly by the flame rather than by conduction. However this multi-layer construction is very difficult to control and will often result in cold spots on the surface of the radiant burner.